This invention relates to the production of carbon black which is pelletized with water and dried before use. A specific aspect of this invention is the improved control of the properties of carbon black which impart stiffness to a rubber compound in which it is used, without the concomitant reduction of the abrasion resistance of the compound.
Rubber is commonly compounded with a quantity of a grade of carbon black as a reinforcing agent. For many uses, the strength or abrasion resistance of the compound, or both, would be inadequate without the carbon black. Carbon black imparts to the compound tensile strength, stiffness, abrasion resistance, and improved extrudability of the uncured material, each to a degree depending upon the properties of the carbon black used.
Stiffness of rubber compounds is commonly measured as the force per unit area of cross section of a compound specimen, prepared under standard conditions, which elongates the specimen by 300% of its original length. This force is designated "300% modulus" in the trade.
While carbon black of one kind or another has been produced since ancient times, most present production of carbon black is carried out in what has come to be known as the furnace processes. In this process a stream of hot gases is generated, usually by burning a hydrocarbon fuel, and a feedstock hydrocarbon is injected into the hot stream of combustion gases, to be converted into a carbon black and product gases. Many variants of the generalized apparatus are known and used commercially, one example of which is disclosed in U.S. Pat. No. 4,106,912.
A large percentage of the carbon black produced by the furnace processes is used to reinforce polymers, particularly rubber used in the manufacture of automotive tires. There are two chief classes of carbon blacks used in making tires, the so-called "soft" blacks used in carcasses of tires where abrasion resistance is not maximized, and the "tread" blacks used in the treads, where maximum abrasion resistance is required, consistent with processability of the stock, acceptable levels of stiffnes or modulus, and extrudability.
Two conspicuous properties of furnace carbon blacks are "surface area" and "structure". Surface area may be measured by adsorptive tests, such as the iodine adsorption test, published by the American Society for Testing and Materials (ASTM) as Method D1510-76. Some estimate of the relative surface area may be obtained by the tint test, in which the ability of the tested black to cover a white pigment is compared to that of a standard carbon black. Such a test is published by the ASTM, as Method D3265-76a. The structure of carbon black refers to its habit of forming, to varying degrees, chains or clumps, which, when viewed in an electron microscope, appear as more or less spheroidal particles fused together. Some structure may appear as fragments of chains of beads grown together; others may suggest clusters of imperfect spheres fused together at the edges. The degree of structure is measured in the industry by the torque on a blade stirring a carbon black while dibutyl phthalate is added, in a standard mixing chamber. The more dibutyl phthalate required to attain a predetermined torque, the greater the degree of structure of the carbon black is deemed to be. This test is published by ASTM as Method D2414-76.
Over a broad spectrum of types of carbon black, the reinforcement of rubber compounds by carbon black increases with its surface area. The abrasion resistance of a compound is thus enhanced in a general way by increased surface area, and to a degree by higher levels of structure. The abrasion resistance imparted to a rubber compound by carbon black at fixed levels of both surface area and structure will vary depending upon the conditions of manufacture of the carbon.
When a tread rubber compound is extruded in the shape suitable for curing in place on the carcass of a tire, the extruded piece must retain dimensional stability and an acceptably regular surface during handling and curing. These properties of the extruded piece depend largely upon the structure level of the carbon black in the compound. Structure also has a major influence upon the modulus of the rubber into which it is compounded; modulus tends to rise rapidly with structure. It is, therefore, necessary at times to provide a level of structure for the control of extrusion properties which may be too high for the level of modulus desired. The manufacturer of carbon black must then use a process which will moderate the development of modulus below the level which would otherwise be attained by the degree of structure required for optimum extrusion properties, yet without any deleterious effects on abrasion resistance.